Maybe it is just the submission packaging. Too many public; the function could be static. Especially painful seems to me using a StringBuilder. You could shift a bit with << and >>.
–
Joop EggenFeb 26 '13 at 15:45

4 Answers
4

Output one line with one integer, the number we get by reversing the binary representation of N.
(emphasis added)

You're outputting two lines, Enter A Number: and the integer. The text Enter A Number:\n11\n is not equal to the text 11\n. If they're using an automated system to check the programs, it could be failing your submission for that reason.

(I haven't checked your code other than that, so there may be other issues as well -- but that one stands out immediately.)

If you know the max size of the binary number (4 bits in your example) and it is 8 bits or less, then the fastest way to perform the reversal is with a lookup table using the current binary value as the index.
For example,

Returns the value obtained by reversing the order of the bits in the two's complement binary representation of the specified int value.
Returns:
the value obtained by reversing order of the bits in the specified int value.
Since:
1.5